1. Field of the Invention
This invention relates to processes for preparing saponified ethylene-vinyl acetate copolymer, and more particularly, to such processes in which saponification is carried out upon finely divided ethylene-vinyl acetate copolymer particles dispersed in an aqueous medium.
2. Description of the Prior Art
It is well-known that high molecular weight thermoplastic resins, for example, polyethylene and ethylene copolymers, can be converted to dispersions of spherically shaped particles which are substantially devoid of particles greater than 25 microns in diameter and in which the number average particle diameter is less than about 10 microns. Thus, McClain U.S. Pat. No. 3,422,049 teaches that such dispersions of finely divided particles can readily be prepared by agitating the molten resin in admixture with water at elevated temperatures and at autogenous pressure, in the presence of certain dispersing agents which are particular block copolymers of ethylene oxide and propylene oxide. The non-agglomerated spherical particles can be recovered as powders by cooling the dispersions below the fusion point of the resin and collecting the suspended material by filtration or centrifugation.
U.S. Pat. No. 3,418,265 further teaches that the particle size of such thermoplastic resin dispersions can be reduced still further, to the sub-micron level, while retaining the unique spherical particle shape by including in the disperion process a volatile, inert, water-insoluble organic liquid that is soluble in the thermoplastic resin in an amount between 0.5 and 20 parts per 100 parts of the resin, whereupon stable, aqueous film-forming latices of high molecular weight polyethylene are formed. Such dispersions can also be formed by including a liquid vinyl monomer such as styrene in the dispersion process.
Although the foregoing dispersion procedures are conveniently operated as batch processes, it is also known to produce such finely divided powders in a sequential, continuous dispersion process. See, e.g., U.S. Pat. No. 3,432,483.
U.S. Pat. No. 3,586,654 teaches that it is further possible to conduct the dispersion process in such a way that the polymer particles can be further transformed into spherical particles of controlled average size and size distributions which are the same, larger or smaller than the starting particles. If desired, the dispersion process can be modified in such a manner as to produce spherical foamed particles (U.S. Pat. No. 3,472,801), or to incorporate within the particles certain colorants (U.S. Pat. No. 3,449,291) and pigments (U.S. Pat. No. 3,674,736).
The fine powders are, by virtue of their small particle size, narrow particle size range, and spherical particle shape, unique states of matter which cannot readily be prepared by other conventional processes known in the art. The advantages and utility of such fine powders has been described in many of the aforesaid patent disclosures. In addition, it has been found that various substrates can be coated by applying the above described dispersions of polyolefin fine powders in an inert carrier, heating to evaporate the carrier, and fusing the polyolefin to the substrate (U.S. Pat. No. 3,432,339). Further, U.S. Pat. No. 3,669,922 teaches a process for preparing colored polymer powders having controlled charge and printing characteristics of value as toners in electrostatic printing.
The hydrolysis of ethylene polymers containing hydrolyzable copolymerized units, for example, vinyl acetate units, is also known in the art. Usually, the hydrolysis of such polymers is accomplished by the process of alcoholysis or transesterification in an alcoholic medium in the presence of an acid or basic catalyst, whereby the acetic ester of the alcohol employed is recovered as a by-product. Solution alcoholysis processes, i.e., in which the reaction is carried out in a mixed solvent of an aromatic hydrocarbon and lower primary alcohol, require the use of large volumes of solvent and larger reactors or kettles. The finished product must be recovered from solution and washed with additional alcohol. The time required to dissolve the resin before hydrolysis is long and the recovery procedure is also time consuming. In addition, the solvent-alcohol mixtures must be separated and recycled for re-use. Solvent losses are incurred which are undesirable from the view point of air pollution as well as for economic reasons. The inherent fire hazard in any process involving organic solvents is a further disadvantage of solution alcoholysis and, additionally, solvent shortages can also present problems.
A different process in which the ethylene-vinyl acetate copolymer in the form of pellets is hydrolyzed by alcoholysis in the swollen solid phase, in a substantially similar alcoholysis media, is described in U.S. Pat. No. 3,510,463. Pressure alcoholysis in the presence of sodium hydroxide in hexane-methanol and methanol alone at temperatures up to 230.degree. C. has also been disclosed. (Japan. 70 33,065, Oct. 4, 1970; Chem Abstrs. 74, 32375v and Japan 70 33,066; Chem. Abstrs. 74, 54620y.)
Less commonly, the hydrolysis of ethylene-vinyl acetate copolymer is accomplished by the process of saponification, for example, in an aqueous emulsion where at least a stoichiometric amount of a strong base such as sodium or potassium hydroxide is required for the hydrolysis of a given number of moles of combined vinyl acetate units in the copolymer. See, e.g., U.S. Pat. No. 2,467,774. In general, however, saponification of ethylene-vinyl acetate copolymers is a slow process at ambient temperatures. Thus, Davies and Reynolds, J. Applied Polymer Sci. 12, No. 1, 47 (1968), have reported that at 25.degree. C., 24 hours is required to effect an 18.1% saponification of an ethylene-vinyl acetate copolymer containing 50.8 weight percent vinyl acetate.
German Democratic Republic (DDR) Patent No. 88,404, describes a process for simultaneously dispersing and saponifying EVA copolymers employing sodium hydroxide or potassium hydroxide as the saponification agent and an alkyl sulfonate, an acyl derivative of N-methyltaurine, a higher fatty acid soap, an alkaryl sulfonate or a nonionic surface-active agent derived from ethylene oxide as the dispersion agent. The process described involves saponifying EVA copolymers at elevated temperature and pressure including, as the final step, discharging the reaction mixture at the operating temperature and pressure directly into a quench vessel at atmospheric or subatmospheric pressure. The quench vessel contains water that is stirred during the discharge operation and the rate of discharge of the reaction mixture is regulated by means of a needle valve. Thus, the sudden release of the reaction mixture causing a portion of the reaction medium to vaporize apparently results in formation of the dispersion due to the atomizing effect of the needle valve. This patent also discloses the optional use of dispersants, but it is apparent from the data provided that such dispersants have only a secondary effect, the primary determinant of dispersion being the discharge of the hot reaction mixture to the quenching bath. From the particle size distribution data provided in the disclosure, it is clear that the presence of dispersing agent seems to favor smaller particles, but is not absolutely essential since comparable dispersions are obtained when dispersing agents are not present in the reaction mixture. There is no indication that a dispersion of the polymer occurs in the reaction mixture prior to discharge when dispersing agents are present but the data provided shows that, on discharge, a dispersion is produced in the presence or absence of dispersing agent. Attempts to obtain dispersions of saponified EVA using N-oleoylsarcosinate as dispersing agent by merely cooling the reaction mixture without the described discharge step of DDR Pat. No. 88,404 have not produced dispersions. Similarly, when arylsulfonate dispersants are employed in lieu of the sarcosinate, no dispersions are obtained when the reaction mixture is cooled. Thus, it must be concluded that dispersion only occurs on discharge. The dispersed product obtained by the method of DDR Pat. No. 88,404 is of fairly large particle size, the heavy majority of the particles being of diameters greater than 0.125 mm, i.e., usually over 80% of the dispersed particles. In addition, the product is composed of irregular particles, with no spherical particles being observed.
The aforedescribed disadvantages of the process of DDR Pat. No. 88,404 have been largely overcome by the simultaneous dispersion and saponification process of McClain copending patent application Ser. No. 824,934 filed July 15, 1977 wherein EVA copoymer is dispersed and at the same time saponified in an aqueous dispersion medium containing an alkali metal soap together with an alkali metal hydroxide in an amount sufficient to saponify the copolymer to a predetermined degree. In addition to its relative simplicity, this process consistently provides saponified EVA copolymers in which the majority of particles is less than 250 microns, and the process can be readily operated so as to provide substantially spherical particles. However, using this process it is ordinarily difficult to achieve a particle size distribution of saponified EVA which will fall within a pre-set range without at least some preliminary, albeit simple, testing since saponification takes place at the same time the dispersion is being formed. Accordingly, there has heretofore existed a need for a simple and convenient process for obtaining saponified EVA particles, desirably spheroid in shape, which will predictably fall within a given range of particle size.